Agrobacterium Tumefaciens and Its Effect on Plants

Introduction

• Agrobacterium Tumefaciens: A Gram-negative bacterium found in soil that affects plants by transferring its genetic material
• Impact on Plants: Causes diseases, specifically affects dicot plants and not monocots
• Relevance: Important for M.Sc. and Ph.D. entrance exams
• Type of disease: Causes Crown Gall disease in plants

Mechanism of Infection

• Genetic Transfer Process: Agrobacterium transfers its genetic material to the plant cell via a conjugation-like process
• Involved Plasmid: TI (Tumor-Inducing) plasmid
  • TI Plasmid Components: Contains T-DNA which is transferred to the plant
  • Role of T-DNA: Contains genes that induce tumor formation (Crown Gall disease)

Plasmid and Gene Structure

• Plasmid Components:
  • Ori Region: Origin of replication
  • Virulence Region: Essential for infection
  • T-DNA: Transferred to the plant, contains genes for cytokinin, auxin, and opine synthesis
• T-DNA Features:
  • Borders: Right and left borders help in the transfer process
  • Gene Content:
    • Auxin genes: Promote cell division
    • Cytokinin genes: Also promote cell division and tumor formation
    • Opine synthesis genes

Virulence Genes and Their Roles

• Key Vir Genes:
  • VirA: Acts as a sensor kinase, recognizes phenolic compounds (acetosyringone)
  • VirG: Response regulator, activates the transcription of other vir genes
  • VirD1/D2: Nicking at the borders, essential for T-DNA processing
  • VirB complex: Forms a channel for T-DNA transfer
  • VirE2: Binds to T-DNA, essential for nuclear localization
  • VirE3, VirD4, etc.: Assist in various steps of the transfer process

Transfer Process

• Step-by-Step Process:
  1. Phenolic Signal: Wounded plant releases phenolic compounds (acetosyringone) which are detected by VirA
  2. Phosphorylation Cascade: VirA autophosphorylates and transfers phosphate to VirG
  3. Gene Activation: VirG activates other vir genes
  4. T-DNA Processing: VirD1/D2 cut the T-DNA at the borders
  5. Formation of T-DNA Complex: T-DNA binds with VirD2, becomes single-stranded
  6. Channel Formation: VirB complex forms a channel (T4SS) for T-DNA transfer
  7. Transfer to Plant Cell: T-DNA is transferred to the plant cell through this channel
  8. Nuclear Transfer: VirE2 and other proteins help T-DNA reach the plant nucleus
  9. Integration: T-DNA integrates into the plant genome via non-homologous recombination
  10. Tumor Formation: Integrated T-DNA induces tumor formation by high cytokinin and auxin production

Additional Information

• Other Agrobacterium species:
  • Agrobacterium Rhizogenes: Causes hairy root disease, uses RI (Root-Inducing) plasmid
• Role of Opines: Synthesized by plant cells to be used by bacteria

Summary

• Agrobacterium Tumefaciens uses a sophisticated mechanism involving vir genes and T-DNA to infect plant cells and cause Crown Gall disease. This involves a well-orchestrated series of events leading to the transfer and integration of bacterial genes into the plant genome, resulting in tumor formation.